How much water evaporates from the world’s rivers and streams? Mechanistic modeling of evaporative losses and ecological consequences

• Applicant: Professor Nima Shokri
• Subject Area: Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 537303427

The global network of rivers and streams spans over 80 million kilometers and is the key to human settlement, supporting diverse aquatic ecosystems, and transporting sediment and carbon into the oceans. Evaporative losses from these important aquatic arteries affect not only water availability but may influence chemical and biological processes of river ecosystems through changes in energy budget and water temperature. Despite progress in quantifying evaporation from other terrestrial compartments, estimates of evaporation from rivers remain largely local and empirical lacking predictability to varying fluvial and climatic conditions at different scales. Specifically, effects of turbulence and mixing of these dynamic evaporating surfaces, aerodynamic interactions with overlying air, variable radiative regimes and flow characteristics have not been systematically incorporated into evaporation models across a hierarchy of scales. In contrast with Penman-type approaches that build on empirical surface exchange coefficients, this project seeks to develop a physically-based framework for constraining river and stream evaporation considering a scale-appropriate energy partitioning scheme. A model of a characteristic river/stream profile (1-D) will be developed to include salient features of mixing and turbulence while accounting for variations in radiative energy inputs and flow characteristics. We hypothesize that the ingredients included in this simplified representation capture the primary processes affecting evaporative losses and water temperatures in a scalable manner while reducing empiricism. Our specific objectives are to: i) develop a mechanistic framework that incorporates hydro-aerodynamic characteristics with radiative energy partitioning and heat budget of characteristic river profiles to quantify evaporative losses over stream stretches, ii) conduct systematic laboratory and field experiments to identify dominant fluvial characteristics and environmental factors that govern evaporation dynamics to guide model evaluation and upscaling, and iii) classify spatio-temporal characteristics of global fluvial network to guide upscaling of the theoretical approach. The project will harness laboratory and field equipment and expertise at Desert Research Institute (USA), Jülich Research Centre (Germany), Utrecht University (Netherlands), and Boston University (USA). The quantitative framework will enable assessment of future climate effects on stream characteristics (early snowmelt, increasing air temperature, changes in rainfall) with important ecological consequences. The project will provide improved estimates of evaporative losses from global fluvial network towards addressing knowledge gaps concerning water balance and availability in arid regions; and improving the science underlying regional and transboundary water transfer and water rights.

• DFG Programme: Research Grants
• International Connection: USA
• Co-Investigators: Dr. Milad Aminzadeh, Ph.D.; Professor Dr.-Ing. Peter Fröhle
• Cooperation Partner: Professor Dr. Dani Or